Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by a layer of fluid. Sometimes, multiple layers of a coating are applied on top of each other. After the deposition of a coating, it can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure sensitive adhesive will not aggressively stick. Methods of applying coatings are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vorstrand Reinhold Publishing Co., New York 1984.
Die coating methods include extrusion coating, slide coating, and curtain coating. In die coating, the web to be coated is usually supported by a precision back-up roll. Coating streaks are a problem in any die coating system. A coating streak is a line of material that is uncoated or has a coating thickness less than the average coating thickness within a coating. A coating streak is caused by something blocking or disturbing the flow of fluid in the coating bead which spans the gap between the coater die and the web. Streaking is a major cause of waste in many coater lines. There are three major bead disturbances in the coating bead exiting from the die which cause streaking.
Nicks or dents in the die and dirt particles from the coater area are two disturbances that alter the bead. Dirt particles could be carried by the web and lodge in the gap between the web and coater die, and dirt in the fluid might get lodged in the die and disturb the flow.
The third bead disturbance is caused when the coating fluid dries on the lip and in the feed slot of the coater die, disrupting the precision geometry of the die and thus disturbing the fluid flow. Fluid drying is particularly prevalent when coating fluids with high volatility solvents such as tetrahydrofuran and methyl ethyl ketone are used. Also, drying is more prone to occur when the coating is intermittent or interrupted, such as where separate patches are coated on a web or where discrete piece parts are coated such as by pulsing.
Typically, die coaters do not include any system for controlling the drying of coating fluid on the die lips. U.S. Pat. No. 4,292,349 describes a shield to cover a coating die which collects the solvent evaporating from the fluid itself as it is coated. Eventually, the solvent concentration builds up enough to suppress drying and thus streaks. This patent describes a passive means of suppressing the drying using shields to collect the solvent as it dries from the coating. This method can retard the drying on the coater lips but cannot eliminate it. Additionally, this method only prevents the drying of fluid of the top (downstream) lip of the coater and does nothing to retard drying on the bottom (upstream) lip of the coater.
PCT International Publication No. WO 90/01178 describes a shield for a cascade coater (slide or curtain) to prevent disturbances to the fluid as it flows down the slide.